Asked and answered correctly.. That was perfect!
The flex plate on both the 304 and 360 have a dual bolt pattern so that you can bolt them up to a 904 or 727. So, if you are trying to find a flex plate for a 304 that bolts up to a 727, the stock 904 flex plate is the same thing.
There are pictures of them in this thread.https://rebsamcandjeep.proboards.com...els-flexplates

I replaced the seized 360 in my ‘73 J4000 with a ‘77 AMX 304. I discovered there are 3 different flexplates for 304 in Jeeps (and one different one in ‘71 only AMC cars with 304 and stock Borg Warner Auto) Early TH400 (nailhead Version with AMC adapter ring and crank spacer that fits the ‘71 only 304 and 72 to ‘79 304 as well. Same 304 flexplate balance until AMC patterned TH400 in ‘74) Torque converter was different too.

‘72 to ‘79 304 Works with amc TH400 case and later flexplate.

Last but not least ‘72+ AMC car and ‘80+ CJ with 304 auto have the correct Flexplate for 727 or 999. You can upgrade a bit further depending how far you want to go. I got to the point that I “hate” the Ford style starter and would rather score an ‘87+ 727 with XJ style starter and have the later flexplate rebalanced for the 304.
I had planned to put my 304 with an AW4 ($100 to rebalance 6 cyl flexplate) and run a Mopar Dana/Spicer 20 TCase off an IH behind it.

Thanks for that...Now I'm totally confused...
I checked the serial number and it is a '72-80. The rest of the stuff the seller had was all Jeep stuff, so I assume it came from a CJ. The starter is Motorcraft. It had a flywheel on it, but when I said it was going in a Wagi with a TF727 he kept it. The engine is complete with the harmonic balancer.
So, which flexplate should I be looking for? Do I just search for the year-range?

The 304 and 360 have the same crank shaft. They are just balanced differently.
The 1970 & 1971 crankshaft has a flat flange and will not work with a 727/904 flex plate unless modifications are made.All1972 and later 304/360 crank shafts will work with the 727/904 flex plate.

Thanks for that...Now I'm totally confused...
I checked the serial number and it is a '72-80. The rest of the stuff the seller had was all Jeep stuff, so I assume it came from a CJ. The starter is Motorcraft. It had a flywheel on it, but when I said it was going in a Wagi with a TF727 he kept it. The engine is complete with the harmonic balancer.
So, which flexplate should I be looking for? Do I just search for the year-range?

Thanks,
Shawn

Your best bet would be to find a used one. The counter weight size is noted in the post. To do it right, the new flex plate should be match balanced to the original flywheel. You could buy one of these but more than likely the engine will vibrate unless you get it match balanced.https://www.summitracing.com/parts/p...rs?prefilter=1

Or, you could have a machine shop match balance your 360 flex plate to the 304 flywheel.

LOL, this reminds me of the American Treaure films with Cage & Voight...One clue leading to another clue, to another. But in both of those, Cage's character was heavily financed, went through a nasty divorce but did get the treasure (and win the girl, again).
Is there anyway to further pinpoint the year of the engine? 70-82 is a big spread. I did see a big S stamped next to the serial number, but clearly not part of it. Does it have any meaning? Is it a secret ancient Mayan code?
I do appreciate the links and comments. I did a quick search for used flexplates: Some are awfully proud of theirs, yeesh.
Thanks,
Shawn

LOL, this reminds me of the American Treaure films with Cage & Voight...One clue leading to another clue, to another. But in both of those, Cage's character was heavily financed, went through a nasty divorce but did get the treasure (and win the girl, again).
Is there anyway to further pinpoint the year of the engine? 70-82 is a big spread. I did see a big S stamped next to the serial number, but clearly not part of it. Does it have any meaning? Is it a secret ancient Mayan code?
I do appreciate the links and comments. I did a quick search for used flexplates: Some are awfully proud of theirs, yeesh.
Thanks,
Shawn

Yes there is but it really doesn't matter. From 1972 to 1982 they are all the same. There are casting numbers and the year embossed on the cylinder head under the rocker arm cover. I didn't find a Pioneer brand that was just for a 304. The ones I saw, listed the same part number is for a 304 and 360. Again, the best bet is to have your 360 flex plate match balanced to the 304 flywheel if you can borrow the flywheel from the guy. Post up a picture of the back of the 304 crank and we can tell you if it is a 1972 or later engine.

since 304/360 crank(s) are same I imagine an off shelf flexplate probably be fine.
a new rebuild obviously best to have balanced but for a swap probably be fine to just buy/use flexplate ya get.

No, it doesn't work that way. Each engine was balanced individually. Each vibration damper and flex plate was balanced to the rotating assembly one engine at a time. Even when you swap a flex plate from one 360 to a different 360 you will end up with a vibration. Some of the vibration my not be noticeable. Some will vibrate so bad that you swear the the engine is missing.
This is the SAE paper explaining how they balanced the engines.IMG_0938 by Rick Jones, on Flickr

*IF* original to engine flexplate/flywheel/balancer is even available...
having a shop match the balance is best
many times they aren't so short of pulling engine apart to balance rotating assembly I would just slap a replacement in there and call it good.

if a guy is rebuilding and engine balancing is a no brainer
if a guy is just swapping a runner engine in then it'll likely be fine with a replacement.